Recently there has been considerable work on the transcutaneous monitoring of blood gases, and several firms are offering instruments for clinical monitoring. However, the variables which affect the correlation between arterial gas tensions and transcutaneous readings are only poorly understood. Our studies are aimed at providing basic information on transcutaneous gas movement and how it is affected by variables such as blood flow. Inert tracer gases such as helium are being used to help us understand how these variables influence the observed readings.